The future development of advanced neuroprosthetic systems is likely to significantly improve the quality of life for persons who suffer from a variety of disorders, including those who are deaf, blind, or paralyzed, etc. Additionally, the development of apparatus and techniques for discretely sensing localized nerve impulses within neural tissue promises to provide new avenues for research and treatment of neurological disorders. However, the development of such advanced neuroprosthetic systems and sensing apparatus will be dependent upon the availability of microelectrode arrays which may be implanted into nerves for the purpose of providing reproducible, localized stimulation or sensing at discrete locations.
One example of an area where advanced neuroprosthetic systems may be of great benefit is in the treatment of hearing disorders. At present, devices known as cochlear implants are being used to restore varying levels of functional hearing in persons who suffer from certain types of hearing loss. The cochlea of the ear is a spiral-shaped, fluid-filled structure that is lined with auditory sensory cells known as “hair cells” which move in response to sound, thereby stimulating the adjacent auditory nerve. The cochlear electrode array resides within a region of the cochlea known as the scala tympani and, thus, is referred to as an “intrascalar electrode.” Such intrascalar electrode delivers electrical impulses that bypass the hair cells and stimulate the adjacent portion of the auditory nerve. However, the typical intrascalar electrode is located relatively far from the auditory nerve and is separated from the nerve by the impedance of the modiolar wall. Thus, the spatial resolution of the stimulation currents that each the auditory nerve is relatively low. This lack of spatial resolution limits the number of independent information channels that can be used to transfer auditory information through the auditory nerve to the brain. Moreover, relatively high threshold currents are needed by the intrascalar electrodes, thus resulting in high power consumption which affects the batter life of cochlear implants.
An alternative to the use of intrascalar electrodes is direct stimulation of the auditory nerve by way of an intraneural electrode array that is actually positioned within the auditory nerve. The use of an intraneural electrode array can substantially increase the number of functional channels and by increasing the selectivity and dynamic range of each stimulating electrode. It is believed that, at least in some patients, more accurate tonotopic representations may be obtained if an electrode array is placed directly within the auditory nerve instead of in the scala tympani of the cochlea. Direct stimulation of the auditory nerve may also offer increased spectral resolution and lower power consumption when compared to cochlear implants. The possibility exists to significantly improve human auditory prostheses by Simmon performed the early intranerual electrode implantations, but the relatively large size of the platinum-iridium wire electrodes did neither permit atraumatic insertion, nor accurate placement of these electrodes.
Early attempts in developing intraneural electrodes were based on platinum-iridium wire electrodes, which led to insertion trauma and reduced placement accuracy. In recent years, the development of Microelectromechanical Systems (MEMS) technology (sometimes referred to as Micro Systems Technology or “MST”) has made it possible to replace bulky off-chip components with microfabricated counterparts. Using MEMS technology, a number of researchers have fabricated microelectrode arrays intended for implantation in the central and peripheral nervous systems. However, even with the use of MEMS fabrication techniques, certain issues relating to electrode size, the need for electrical wires to communicate and transfer power to the arrays, and the need for hand assembly have remained largely unsolved.